A fuel cell system includes a fuel cell which generates a power by an electrochemical reaction between a fuel gas and an oxidizing gas. In a solid polymer type fuel cell, in order to increase the efficiency of the power generation thereof, an electrolyte film needs to be held in a humid state. This humid state is usually held by a humidifier which humidifies the fuel gas or the oxidizing gas.
In a conventional humidifier, an internal humidifying system which performs water exchange between a highly humid oxidizing off gas and a lowly humid oxidizing gas is frequently employed. The oxidizing off gas is more humid than the oxidizing gas, because the oxidizing off gas discharged from an air electrode of the fuel cell contains water generated by the electrochemical reaction.
JP 2005-44665 A discloses a humidifier of the internal humidifying system which includes a hollow fiber membrane having steam permeability in a case thereof. In this humidifier, the upper part of the case is provided with an inlet and an outlet for the oxidizing gas, and both the side parts of the case are provided with an inlet and an outlet for the oxidizing off gas. In addition, the oxidizing off gas and the oxidizing gas are introduced into the case, and the respective gases flow outside and inside the hollow fiber membrane. In consequence, a water is absorbed from the highly humid oxidizing off gas by the hollow fiber membrane, and the water passes through the membrane by a capillary phenomenon to move to the lowly humid oxidizing gas, whereby the oxidizing gas is replenished with the water.